Conventionally, a synchrotron using a high-frequency voltage generated from a high-frequency accelerating cavity has been used to accelerate a charged particle beam. In recent years, a method for accelerating a charged particle beam by using an induced voltage generated by an induction accelerating cell has been developed. The accelerated charged particle beam has been used for a physical experiment, a medical treatment, and the like.
In the synchrotron, a charged particle beam is circulated along a design orbit while undergoing betatron oscillation. Conventionally, in order to extract an accelerated charged particle beam, “third resonance”, which is a resonance phenomenon occurring in the horizontal direction (referred to as the lateral direction, the radial direction, the x direction, or the like) of the charged particle beam, has been used as described in the prior art of Patent Literature 1.
Here, the “horizontal direction” of a charged particle beam means the direction in which the direction of increasing the radius of the circulation surface of the orbit of the charged particle beam in the synchrotron is defined as the positive direction. When the charged particle beam circulates in the synchrotron, and when the circulating orbit of the charged particle beam is observed from above the circulation surface, the centrifugal force acts on the positive side in the horizontal direction. For this reason, an extracting port of the charged particle beam is generally arranged on the outer side of the circulating orbit so as to prevent the circulating beam line and the extraction beam line from interfering with each other. At the time of extracting a charged particle beam, a part of the charged particles circulating on the outermost side, among the circulating charged particles, are extracted.
Conventionally, in order to extract a charged particle beam from its circulating orbit by using the third resonance, there has been used one of the following methods: (1) changing the energy of the charged particle beam in its traveling direction; (2) making the charged particle beam oscillate at a resonance frequency by using a lateral high-frequency electric field; and (3) changing the stable region of betatron oscillation by exciting a six pole magnet.
Here, the magnetic field generated by the six pole magnet defines the size of the stable region in the horizontal phase space of the charged particle beam. As the intensity of the magnetic field is increased, the size of the stable region is reduced. Further, the phase space can be represented as a plane in which the relative position of each charged particle with respect to the center of the design orbit drawn by a reference particle is adopted as the abscissa, and in which the momentum deviation of each charged particle is adopted as the ordinate. The phase space is generally used to explain the behavior of a charged particle beam
It is known that the condition that a charged particle beam can be retained in the stable region is influenced both by the coordinates of each charged particle in the horizontal phase space and by the momentum deviation of each charged particle in its traveling direction. That is, it is known that, even among charged particles located at the same coordinates in the phase space, when the momentum deviation of a charged particle in its traveling direction is large, the stable region of the particle in the phase space is small.
For example, an invention corresponding to the charged particle beam extraction method (3) is disclosed in Patent Literature 1. The invention described in Patent Literature 1 is featured in that the emittance is increased before the start of extraction of a charged particle beam (claim 1). Further, the invention described in Patent Literature 1 is featured in that the emittance in the horizontal or perpendicular direction at the start of extraction of a charged particle beam is substantially fixed without depending on the energy of the beam (claim 2). Further, the invention described in Patent Literature 1 is featured in that the emittance in the horizontal or perpendicular direction is substantially fixed during a period from the time of start of acceleration or from the middle of acceleration to the end of acceleration (claim 3). The principle of the invention described in Patent Literature 1 is shown in FIG. 8. Here, the emittance means a “phase space spread” of a charged particle beam resulting from the momentum error of each of actual charged particles with respect to the motion trajectory of an ideal charged particle.
FIG. 8 is a schematic view which shows the principle of a conventional charged particle beam extraction method of extracting a charged particle beam by changing the energy of the charged particle beam and in which the ordinate represents the momentum deviation (Δp/p) in the charged particle beam and the abscissa represents the time (t).
The momentum deviation (Δp/p) in the charged particle beam means a ratio of a deviation (Δp) between the traveling direction momentum of a charged particle (reference particle) having the energy to circulate along the design orbit and the traveling direction momentum of each of charged particles, with respect to the traveling direction momentum (p) of the reference particle. Reference character T0 denotes a time period (circulation period) required for the reference particle to circulate along the design orbit once in the synchrotron.
As shown in FIG. 8, in the conventional charged particle beam extraction method, a high-frequency voltage 6e (solid line) is applied, from a high-frequency generation apparatus provided separately from a high-frequency accelerating cavity, to an entire charged particle beam 6 (dotted line) accelerated by a high-frequency voltage 2c (dotted line) generated from the high frequency accelerating cavity, so that an entire charged particle group 6a (solid line) is further accelerated (in the direction shown by the upward arrow) just before the charged particle beam is extracted. Thereby, a charged particle group 6b (colored portion) deviated from the stable region to a non-stable region 8a is selectively extracted to an emission line 4, for example, by an emission deflector.
In any of the conventional charged particle beam extraction method, including the technique described in Patent Literature 1, the accelerated charged particle beam as a whole is influenced as shown in FIG. 8. Further, much time is required for adjusting the charged particle beam to the extraction state. Further, the intensity of the extracted charged particle beam is greatly varied, so that it is difficult to fix the beam intensity.
In the conventional charged particle beam extraction method, the beam intensity [A] of the extracted charged particle beam cannot be fixed due to the following reasons.
The conventional synchrotron using a high-frequency voltage (RF) is based on the premise that the high-frequency voltage performs both the function of confining the charged particle beam in its traveling direction and the function of accelerating the charged particle beam. Further, as for the horizontal resonance frequency, even when a state of fixed magnetic field intensity is to be created by the magnetic field intensity of a deflecting electromagnet and by the magnetic field intensity of a converging electromagnet, the magnetic field intensity is changed in a range of the order of 10−4 (represented by the double arrow between the solid line and the dotted line in the stable region 8a in FIG. 8) under the influence of power supply noise, and the like.
Further, the extraction condition of a charged particle beam is based on the condition that the energy error (momentum deviation) of the charged particles in the charged particle beam is a fixed value or more, that is, on the condition that the charged particle beam circulates away from the design orbit in the horizontal direction, and is also based on the condition that a decimal part of the resonance frequency defined by the magnetic field intensity reaches one third.
Therefore, as the conventional charged particle beam extraction method, one of the following methods are used: (1) a method in which the magnetic field intensity of the six pole magnet is changed in the state where the energy of the charged particle beam is fixed, that is, the charged particle beam is not accelerated, (2) a method in which the charged particle beam is accelerated in the state where the magnetic field intensity of the six pole magnet is fixed, and (3) a method in which the charged particle beam is further horizontally oscillated at a horizontal resonance frequency in the state where the magnetic field intensity of the six pole magnet and the energy of the charged particle beam are fixed.
However, in the above-described extraction methods (1) and (2), since a charged particle beam always exists in a state where the “resonance condition” is barely satisfied, and since the horizontal betatron frequency is also changed according to the change of the magnetic field due to noise, the moment when the condition that the decimal part of the resonance frequency determined by the intensity of the magnetic field reaches one third is satisfied, and the moment when the condition is not satisfied, are determined by noise. Since the noise is not a controllable factor, only a beam intensity having a large temporal variation is obtained by the above-described methods (1) and (2).
On the other hand, in the above-described method (3), the intensity of the charged particle beam can be made constant to some extent, but much time is required to increase the oscillation of the charged particle beam after a high-frequency voltage is applied to change the horizontal resonance frequency of the charged particle beam (Patent Literature 1).
As shown in FIG. 8, in the conventional charged particle beam extraction method, since the entire charged particle beam is accelerated by a low high-frequency voltage different from the high-frequency voltage used in the high-frequency accelerating cavity, a part of the charged particle beam is always in contact with the region of the resonance condition (non-stable region 8a), and the resonance condition are also varied as described above. Further, a large number of times of circulations of the charged particle beam are required to return the charged particle beam from the region of the resonance condition (non-stable region 8a) to the design orbit.
In this way, in the conventional charged particle beam extraction method which is based on the variation of the resonance condition and in which the charged particle beam is extracted due to noise, the extraction condition, that is, the beam intensity of the extracted charged particle beam cannot be controlled to be temporally fixed.
FIG. 9 shows the distribution of charged particles in the phase space in a conventional circular accelerator. On the abscissa x which represents the horizontal direction, and in which the coordinate 0 corresponds to the position of the design orbit, the positive direction side from the coordinate 0 is the outer side of the design orbit, and the negative direction side from the coordinate 0 is the inner side of the design orbit. The ordinate x′ represents the orbit gradient x′=dx/ds corresponding to the horizontal momentum, in which dx represents a lateral position of an actual charged particle beam when the design orbit is used as the origin, and in which ds represents the position of the actual charged particle beam in its traveling direction.
The group of charged particles forming the charged particle beam circulates while undergoing betatron oscillation. In the case where the stable state, in which the charged particle beam continuously circulates in the synchrotron, is continued, when the orbits of the charged particles are plotted in the stable state, each of the charged particles draws an orbit (closed orbit) closed in the phase space.
Here, in the state where no external force is applied to the charged particle beam circulating in the synchrotron, the size of the charged particle beam is increased in the X and Y directions due to the repulsive force between the electric charges of ions. For this reason, beam converging force is generated by a four pole electromagnet so that the charged particle beam can stably circulate in a vacuum duct.
At this time, the charged particle beam circulates in the synchrotron while performing an oscillation motion which is based on the relationship between the repulsive force and the converging force and which is described by the same equation as the spring motion. Because of the same principle as the principle that a resonance frequency exists in a mechanical structure, the charged particle beam also has a resonance frequency at which, when an oscillation having a specific frequency is given to the charged particle beam, the amplitude of the oscillation is increased with time.
The lateral oscillation frequency during one circulation of the charged particle beam is referred to as tune, and it is known that the resonance condition of the beam is established when the decimal part of the tune is ½ or ⅓ . . . 1/n.
Each of the charged particles forming the charged particle beam has a momentum and a position which are slightly different from those of the other charged particles. When the amplitude of the resonance is adjusted, the timing relationship in the circulating direction of the charged particle beam is not influenced by the adjustment, but the lateral amplitude of only the charged particle satisfying the resonance condition is increased, so that the charged particle reaches the extraction orbit. The resonance amplitude of the charged particle beam is adjusted by establishing the above-described resonance condition by adjusting the current of the four pole electromagnet or the six pole magnet.
At this time, when the charged particle beam is extracted by using the third resonance, since the oscillation frequency of the charged particle beam is different depending on the position and momentum of the charged particle beam, the region (stable region) in which the charged particle beam can be stably circulated, and the region (non-stable region) in which the amplitude of the resonance is increased, are respectively separated into inner and outer regions of a triangle as shown in FIG. 9. The region inside the triangle is the stable region, and the region outside the triangle is the non-stable region. When the charged particle beam is circulated in the state where the decimal part of the tune is slightly larger than ⅓ or ⅔, and when the energy of the charged particle beam is increased while the magnetic field given to the charged particle beam is kept constant, the tune is reduced, and hence the non-stable region in the figure is reduced.
Therefore, in the state where the resonance state is close to the third resonance state, the charged particle beam is moved to return to substantially the same coordinates in the phase space after being circulated three times in the synchrotron, and hence the stable region is made to have a shape close to a triangular shape. Each of the charged particles forming the charged particle beam has a different Δp/p.
The betatron oscillation frequency is determined by the intensity of the magnetic field generated by the deflecting electromagnet and the converging electromagnet which configure the synchrotron, and the size of the stable region is determined by the intensity of the magnetic field generated by the six pole magnet. That is, whether or not a charged particle draws a closed orbit is determined by whether or not the value of Δp/p of the charged particle is a certain value or less, which is determined by the six pole magnet. When the value of Δp/p and the intensity of the magnetic field generated by the six pole magnet are respectively assumed to be certain values, the boundary of the stable region corresponds to the boundary of the triangle shown in FIG. 9.
Note that, since each of the charged particles has a different value of Δp/p, the size of the stable region is reduced when the value of Δp/p is increased, and the size of stable region of a charged particle having a value of Δp/p larger than a certain threshold value is reduced to zero. That is, even in the case where the intensity of the magnetic field generated by the six pole magnet is fixed, when the value of Δp/p of a charged particle becomes larger than a certain value, the charged particle is surely extracted. In other words, the size of the stable region is different depending on the value of Δp/p of each of the charged particles.
In FIG. 9, the inside of a (large) triangle surrounded by boundaries 8d shown by dotted lines is a stable region 8b of charged particles 6d (black dots) under acceleration. When the entire charged particle beam is accelerated with the high-frequency voltage 6e to extract the charged particle beam, the momentum of a charged particle 6c is increased, and the stable region is changed to a stable region 8c (Δp/p=0.003) shown by the hatched region surrounded by a boundary 8f. The size of the stable region 8c is reduced as the value of Δp/p is increased. Here, the sake of convenience, the momentum deviation of the charged particle before acceleration is assumed as Δp/p=0.0026, and the momentum deviation of the charged particle after acceleration is assumed as Δp/p=0.003.
As described above, the charged particle 6d having a small value of Δp/p draws a closed orbit in the stable region 8b and hence is not extracted. On the other hand, the lateral oscillation of the charged particle 6d located in the non-stable region 8a is gradually increased as shown by the broken-dotted line, and hence the orbit of the charged particle 6d is expanded toward the outer side. At a certain part located on the synchrotron orbit, an emission deflector is provided, which generates an electric field only in the region located at a certain position in the lateral direction and which largely deflects the orbit of the charged particle entering the region.
Note that the region (extraction region 8e), in which the electric field is generated, is determined at the time of designing the emission deflector. The charged particle 6d located in the non-stable region 8a enters the extraction region 8e, so as to be extracted into the emission line 4. On the other hand, the charged particle 6c in the stable field 8c continues to be circulated in the synchrotron. When the value of Δp/p of the charged particle 6c is further increased by the high-frequency voltage 6e, the charged particle 6c is extracted.
An accelerator using a high-frequency accelerating cavity for realizing the charged particle beam extraction method described in Patent Literature 1 is shown in FIG. 10 in which a part of the names, reference numerals and characters, and the like, are changed from those described in Patent Literature 1.
As shown in FIG. 10, a conventional accelerator 1a is configured by an injection line 3, a synchrotron 2n, the emission line 4, and a beam utility line 5.
The injection line 3 is configured by a preceding accelerator 3a by which charged particles generated by an ion source are accelerated so as to have a predetermined speed, an injector 3b which injects the charged particles into the synchrotron 2n via a transport pipe 3c, and the like.
The synchrotron 2n is a circular accelerator which accelerates the injected charged particle beam 6 and which emits the accelerated charged particle beam to the emission line 4 by using the third resonance. The synchrotron 2n is configured by deflection electromagnets 2i which maintain the orbit of the charged particle beam 6 on a design orbit 2a inside a vacuum duct, converging electromagnets 2j or divergent electromagnets 2k which are quadrupole electromagnets generating converging force or divergent force, a high-frequency accelerating cavity 2b in which the charged particle beam 6 is confined and accelerated by the high-frequency voltage 2c, a high-frequency voltage application apparatus 2m which applies the high-frequency voltage 6e for accelerating the charged particle beam 6 at the time of extracting the charged particle beam 6, and which is different from the high-frequency accelerating cavity 2b, a six pole magnet 8 which is a multipole electromagnet for resonance excitation, and the like. The charged particle beam 6 is circulated along the design orbit 2a while undergoing betatron oscillation.
The emission line 4 is configured by an emitter 4a, such as an emission deflector, a transport pipe 4b, and the like. The beam utility line 5 is installed at a laboratory or a medical treatment site. The coordinate system is set so that the circulating direction of the charged particle beam 6 is the s axis, the horizontal direction is the x axis (the outer side is set to be positive and the inner side is set to be negative), and the perpendicular direction is the y axis.
On the other hand, in each of Patent Literatures 2 to 6, a technique is disclosed as a charged particle acceleration method using an induction accelerating cell. The technique disclosed in Patent Literature 2 is to provide an accelerating method in which all kinds of ions can be confined and accelerated by using induced voltages which are applied to a charged particle beam from two kinds of induction accelerating cells for confinement and acceleration. Further, the technique is featured in that the preceding accelerator can be eliminated when the extraction energy given to the charged particle beam by the ion source is equal to or larger than the minimum energy required for the circulation of the charged particle beam in the synchrotron.
Patent Literatures 3 to 6 respectively provide a method for controlling the generation timing of the induced voltage applied from the induction accelerating cell, a method for controlling the circulating orbit of the charged particle beam by controlling the generation timing of the induced voltage, and a method for controlling the synchrotron oscillation frequency by applying the induced voltage to the charged particle beam.
Patent Literature 6 provides a method for accelerating the charged particle beam by controlling the generation timing of an induced voltage including positive and negative induced voltages which are applied by a pair of induction acceleration apparatuses and which have a same rectangular shape.
However, a method for extracting the accelerated charged particle beam by using an induced voltage (pulse voltage) generated from the induction accelerating cell is not discussed in Patent Literature 6.
Here, in particle beam therapy, the irradiation dose of a charged particle beam is an amount proportional to the time integration of the particle beam current. The irradiation field of the charged particle beam is a three-dimensional space region to be subjected to radiation irradiation. When the required irradiation dose is irradiated inaccurately or non-uniformly in the target irradiation field, or when the irradiation dose is too much, the probability of occurrence of a serious side effect is increased. On the other hand, when the irradiation dose is insufficient, the probability of recurrence of a tumor in the irradiated part is increased.
That is, in particle beam therapy, it is required to accurately and uniformly perform irradiation of an intended irradiation dose without excess or insufficiency. Therefore, in order to perform irradiation of a high irradiation dose in a short time period, it is very important that the charged particle beam intensity can be temporally controlled.
The charged particle beam extracted from the synchrotron has a diameter of several centimeters. Therefore, in a particle beam therapy apparatus in which a large irradiation area is required, the charged particle beam is irradiated onto a target after the irradiation area of the charged particle beam is expanded in such a manner that the charged particle beam is deflected by using a rotating magnetic field generated by a so-called wobbler electromagnet installed at the extraction beam line 5.
This method is referred to as a wobbler irradiation method. Since the rotation frequency of the rotating magnetic field is 100 Hz or less, when a charged particle beam extraction method, in which the extracted charged beam intensity is not temporally stable, is adopted, the irradiation dose in the irradiation field of therapeutic irradiation needs to be made uniform by irradiating the charged particle beam repeatedly many times while changing the starting point of the rotating magnetic field. Therefore, in the charged particle beam irradiation therapy, the treatment time is increased mainly due to the beam intensity being temporally unstable, which imposes a very large burden on a patient.
Further, as another charged particle beam irradiation method, a method referred to as a spot scanning irradiation method is adopted, in which a required irradiation dose is supplied to a required irradiation part by scanning the charged particle beam on a two dimensional surface similarly to the electron beam scanning in the cathode-ray tube television.
When the irradiation dose is locally changed by the spot scanning irradiation method, the local irradiation dose is determined by the charged particle beam itself. For this reason, when the beam intensity is not temporally stable, the irradiation dose is adjusted by reducing the average beam intensity and by performing the irradiation for a long time. As a result, the burden on a patient is further increased.
Generally, in particle beam therapy, a patient is fixed to a treatment table by using a fixing device having high rigidity in order to improve the irradiation position accuracy. For a medical treatment of a patient whose general conditions are often not good, the increase in the treatment time not only causes a great pain but also becomes a problem which even determines whether or not the irradiation treatment can be applied to the patient.